This invention relates to snap acting pressure switches and more particularly to improvements in the actuating mechanisms for such switches.
The subject invention is particularly applicable to pressure switches of the type used in domestic appliances such as dishwashers, washing machines, etc. which control the water fill tub level for such appliances. However, it will be appreciated by those skilled in the art that the invention may have broader application and may be employed in any type of adjustable switch which senses varying pressures to actuate a switch blade toggle mechanism.
The mechanism for operating switches of the subject invention as contrasted to other mechanisms such as those employed in calibrating the switch are broadly defined as including
(1) a diaphragm for sensing pressures, PA1 (2) a switch blade connected to the diaphragm for completing appropriate electrical circuitry, and PA1 (3) a spring arrangement opposing the diaphragm movement for controlling the trip and reset functions of the switch. Each component thus identified has been plagued with problems peculiar to the function of that component which heretofore has been overcome in varying degrees of success by complex structure, relatively expensive to manufacture and assemble.
Thus, it is commonly recognized that the function of the diaphram is to sense the "head" of water in the tub of the appliance and once the water-fill level is reached, the switch will trip to close an associated water valve and once the level is reduced the switch will automatically reset. However, the water in the tub is subjected to violent agitation during the operation of the machine which can vary the head pressure by as much as 4-6 inches water even though the volume of water in the tub may not materially change. Such transient fluid pressure variations can, in some instances, adversely effect the operation of the switch. Recognizing this problem, the prior art has employed restrictive orifices at the pressure inlet of the switch to "average out" the transient pressures. Because of the small orifice size thus required, nominally 0.020 inch, this has heretofore been accomplished by accurately drilling such hole in a metal piece which was soldered into a metal inlet stem which in turn was soldered to a metal cap that formed the cover for the pressure chamber of the switch. This was a costly operation, both from material and manufacturing considerations. Additionally, the orifice was subject to clogging because of its size and was also subject to damage because of its location.
A frequent problem interfering with the operation of the snap switch is the erosion due to arcing between contact points which in turn occur when controlling highly inductive motor loads such as those encountered in appliance-type motors. Prior art devices have failed to provide simplified structure to compensate for the decrease in contact force as the switch blade moves towards its center trip position. To the contrary the prior art has rigidized the actuator arm of the switch blade and thus the tendency to arc increases as the switch moves toward its center position.
Functional problems have also occurred with respect to the plunger spring arrangement which establishes trip and reset levels of the switch. In one prior art arrangement, a diaphragm post protrudes through a slot in the snap blade and the protruding end serves as a guide for a trip-reset spring which rests directly on the snap blade. Such structure does not provide a stable spring arrangement as the looseness between the post protrusion and spring along with varying angled relationships between spring and snap blade adversely effects the spring forces developed at the low end of the spring. Another spring arrangement utilizes an actuator in contact with the diaphragm post. To permit up and down movement of the snap blade's guide tongue, the diaphragm post is cut with a notch to receive the edge of the opening in the guide tongue. This necessarily results in lost motion between blade and diaphragm posts which adversely effects the determination of a precise trip point of the switch. Additionally, the actuator was free to rotate relative the spring which could adversely effect the switch point because of wear, binding, etc. Finally, complex structure keyed to the actuator has been employed when the plunger arrangement included a second spring which was compressed after some limited travel of the first spring to achieve "proportional" reset of the switch.
Another functional problem is the "contact bounce" which occurs when the movable contact moves from one stationary contact to another and the bounce is caused by the force of impact of the contact closure. A momentary loss of contact occurs during the bounce which may affect the operation of the associated electrical circuit, and arcing may occur between the contacts. Solutions attempted by the prior art include the use of a weaker toggle spring, which reduced the contact force. This reduced the bounce, but introduced other functional problems.
It is thus an object of the subject invention to provide simplified actuating means associated with the plunger arrangement, the snap switch structure and the diaphragm of a pressure switch which overcomes each and all of the above-noted disadvantages.
In accordance with the invention this object is achieved by providing a pressure switch comprising a two-piece plastic housing with a diaphragm mounted therebetween to define an expansible pressure chamber on one side thereof. A snap acting switch blade is positioned on the other side of the diaphragm. The switch blade includes an actuator tongue element carrying a contact movable between a normal and actuated position, and a movable guide tongue element within the actuator tongue operatively connected to the diaphragm for common movement therewith. The first housing portion has a bore aligned with the guide tongue element and a base at the bottom of the bore adjacent the guide tongue. The base has an opening and at least two especially configured, diametrically opposed slots extending therethrough. Adapted to fit within the base for endwise movement therein is a dog-eared, actuator element having a spherical bottom portion which contacts the guide tongue element. A first spring compressed within the bore biases the actuator against the guide tongue element. Additionally, the arrangement may include a second spring normally seated against the base and selectively compressed by the actuator dog-ears after some finite movement of the actuator has occurred. Both springs are stabilized because the fit between the dog-ears of the actuator and the grooves in the base of the bore prevent rotation of the actuator within the bore while also reducing any tendency of the actuator to become cocked within the bore.
In accordance with another aspect of the subject invention, the guide tongue element has a central hole extending therethrough, and the diaphragm post has a spherical end contacting one side of the guide tongue actuator about a circular line removed from the guide tongue's opening. The spherical surface extending from the bottom of the actuator likewise contacts the opposite side of the guide tongue. The guide tongue is thus retained for precise actuation because it is retained between opposing spherical contact surfaces. To assure alignment of the spherical surface, the diaphram post has a blind opening extending within the center of its spherical end which receives a guide stem depending from the spherical surface of the actuator.
In accordance with another aspect of the subject invention, the expansible pressure chamber communicates with a contiguous inlet molded into the plastic housing at the bottom thereof. At the base of the inlet is a wall which contains an especially sized opening which functions as a restrictive orifice for averaging transient pressure changes within the tub due to agitation, etc. The opening, being molded into a plastic part, is necessarily greater in diameter than prior art orifices formed in metal inlets. However, the opening is functionally equivalent to prior art orifices because the length of the opening through the wall is closely controlled at a larger dimension to compensate for the increase in orifice diameter.
In accordance with yet another aspect of the subject invention, the guide tongue element of the snap blade is yieldably deflectable to increase contact force prior to switch trip and thus reduce the possibility of contact point arcing. By permitting the guide tongue to deflect under the loading of the overcenter barrel spring, the vertical attitude of the spring is increased. Accordingly the increased vertical force component of the spring maintains the contacts in firm engagement with one another over a greater switch travel distance prior to trip than that of prior art blades.
In accordance with still another aspect of the invention, the switch blade has a support end attached to the housing, opposite the actuator tongue element, the element being pivotable with respect to the guide tongue at a pivot point spaced apart from the support end. The blade has a relatively flexible portion between the pivot point and the support end to partially absorb the impact forces occurring during contact closure.